For the production of fiber reinforced flat gaskets, which are needed in numerous applications in the technical field, there is used a so called packing sheet calender which has a large diameter heated cylinder, on which the sheet is formed and vulcanized, and a cooled cylinder of smaller diameter which serves as a pressure cylinder for the construction and is usually hydraulically pressed toward the heated cylinder.
The gaskets are thereby produced from a mixture of rubber rendered workable by a solvent and fiber which gives the finished product its strength. Traditionally asbestos fiber was used as the reinforcing fiber since the production and use of the gasket excluded other fibers on thermal grounds. However in recent times asbestos has become increasingly undesirable for reasons of health, and, in so far as possible, is replaced by high strength and thermal resistant synthetic fiber.
On the precision of the roundness of the cylinders, the quality of the cylinder surfaces, the temperature and its exactness, the uniformity of the cylinder pressure and, above all, also the exactness of the speed of rotation of the cylinders (synchronism), the use of synthetic fibers imposes extremely high requirements.
In the production of packing sheets on the calender there are, in addition to the aforementioned machine characteristics, still numerous details which influence the production, for example:
the temperature of the heated cylinder. It is controlling for the vulcanization and thereby for the speed of production of the sheet.
The exactness of the temperature. This is significant with reference to the uniformity of the vulcanization.
The operating speed of the cylinders. It depends on different parameters as for example the adhesion of the mass on the cylinder surface and, depending on the rate of vulcanization and the number of passes under the pressure cylinder in a cycle, determines the compression and quality of the sheets.
The pressure of the pressing cylinders at opposite ends of the pressing cylinder. It is important for the uniformity of thickness of the sheet produced.
The line pressure of the pressing cylinder. It is decisively important for the compression of the sheet.
Correction of cylinder bending. For the production of optimal conditions there is provided a counter bending device which is adjusted to correspond to the pressing force.
The pressing force. It must be optimized with respect to the mixture from which the sheet is formed and the sheet thickness.
The adjustment of circumferential speed of the two individually driven cylinders to one another, which normally must be precisely the same but occasionally may be very slightly different to apply a precisely adjusted friction to the sheet.
The feeding of the mass in relation to the amount and the optimal distribution over the length of the cylinder gap, whereby the uniformity of the sheet tolerance over the working width of the sheet is assured.
An alteration of the parameters after the beginning of the sheet production increasing sheet thickness, for example of the pressure of the pressing cylinder, the speed of vulcanization with decreasing thickness, the number of over-rollings etc.
With the traditional material of the packing sheet, thus with asbestos fiber reinforcing, it was sufficient for most part with reference to carrying out this complicated work process to make the initial adjustment and the running corrections of the production parameters by a service man who also attended to the feeding of the calender and, on the basis of his experience, optimized all other values as required.
By reason of the promotion of asbestos free packing sheets and hence the required introduction of high strength temperature resistant synthetic fibers, working with the mixture has become much more difficult. In particular the discovery and the checking of optimal working parameters has become, in part, so problematic that only specially experienced service people are in a position to produce satisfactory packing sheet. In spite of this, in the production of packing sheets there is much waste which is not commercially acceptable.